Depleting fossil resources spurred interest for sustainably producing polymers, chemicals and fuels indispensable for mankind. Biomass is the one of the viable and renewable alternatives in particular for the production of chemicals and polymers as it is the only renewable source that has carbon. In particular, plant biomass is the valuable feedstock for chemical industry as it contains oils, carbohydrates, lignin, proteins, waxes, and secondary metabolites. Nature produces large amounts of plant biomass through the process of photosynthesis, in which carbohydrates are most abundant. Carbohydrates derived oxygenated chemical platforms have the potential opportunity through diverse synthetic protocols in making energy and chemicals sustainably, necessary for the future generation.
Levulinic acid (LA) is one such building block platform chemical identified by the U.S. Department of Energy; it is prepared from sugars by dehydration followed by hydrolysis via intermediate, 5-hydroxymethylfurfural (HMF). LA is a precursor for the preparation of various value added products, among which γ-valerolactone (Gvl) is an important product obtained through selective hydrogenation. Many research groups after conducting several experiments, concluded that Gvl is a sustainable liquid, which can be used in the production of energy and carbon-based consumer products, as it is easy to handle and store (γ-valerolactone—A sustainable liquid for energy and carbon-based chemicals. Green Chem. (2008) 10, 238-242).
Catalytic hydrogenation of LA to Gvl is an important step in bio refinery because it has several industrial applications such as in biofuels (valeric and liquid alkanes for transportation fuels), additive in biofuel, and as eco-friendly high boiling solvent in particular for biomass conversions and in many organic transformations. In addition, Gvl is a raw material in the production of various chemicals (aromatic hydrocarbons and γ-hydroxy-amides), and polymers (nylon intermediates such as adipic acid, dimethyl adipate and caprolactam). The applications of Gvl are shown in FIG. 1.
Worldwide several research groups are working in developing an efficient conversion protocol for LA to Gvl through hydrogenation using various hydrogen sources like molecular hydrogen, formic acid, and alcohols. However, till date no catalytic process has been reported to be effective near ambient conditions and in shorter time with high yields. Details of some the efficient catalytic systems reported in the prior-art are discussed below.
W. Leitner's research group in their paper titled “Selective and flexible transformation of biomass derived platform chemicals by a multifunctional catalytic system” in Angew. Chem. Int. Ed. (2010) 49, 5510-5514 reported Ru (PnOct3) as an active catalyst for LA conversion to Gvl at 160° C., 100 bar H2 pressure in 18 h in presence of NH4PF6 as additive. Drawbacks of the work are high temperature, high pressure, long reaction time and necessity of additional chemicals. In addition, the complexes are homogeneous and hence not recyclable and poses separation and disposal problems.
L. T. Mika and his research group in their paper titled “Efficient catalytic hydrogenation of LA: A key step in biomass conversion” in Green Chem. (2012) 14, 2057-2065 reported 99.9% yield of Gvl using Ru (acac)3 with bulky phosphine ligands as catalyst at 140° C., 10 bar H2 pressure in 4.5 h. Drawbacks of the work are high temperature and longer reaction time. In addition, the catalyst is homogeneous and hence not recyclable. Moreover, it involves many synthetic steps and use expensive reagents for the preparation of catalyst.
F. E. Kuhn in his paper titled “Catalytic hydrogenation of LA in aqueous phase’ in Journal of Organometallic Chemistry (2013) 724, 297-299 studied the reaction using various water soluble bulkier phosphine ligands along with Ru (acac)3 as catalyst and achieved 99% of LA conversion with 97% selectivity of Gvl at 140° C., 50 bar H2 in 5 h. The main drawbacks are multi-step synthesis of complexes and harsh reaction conditions.
Q-L Zhou et al., in Chinese Patent 102558108 have disclosed the use of iridium trihydride complexes as catalysts for the conversion of LA to Gvl in presence of base and solvent and achieved 96% of Gvl yield. Drawbacks of the work are synthesis of complexes that involves complex multi-step reaction that required controlled conditions, long reaction time (15 h), requirement of additional bases, and use of organic solvents. Further, the complexes are homogeneous and pose difficulties during separation.
To overcome the problems associated with homogeneous catalysts, heterogeneous catalysts were also used for this reaction.
L. E. Manzer in U.S. Pat. No. 6,617,464 B2 disclosed the preparation of Gvl from LA in presence of optionally supported metal catalysts which are preferably selected from the group consisting of Ir, Pd, Pt, Re, Rh, and Ru supported on various supports like carbon, silica and alumina, and the combination of metal and support thereof. He found that Ru on carbon as the more active catalyst at 140 to 160° C., pressure 55 bar H2 in 2 h than other metal and/or supports. The drawbacks of the process are high reaction temperature, high pressure, and necessity of prior reduction of catalyst that require additional energy and raw material.
In another Chinese Patent 101733096 B, a process for the preparation of acid resistant catalysts is disclosed for effective hydrogenation of LA to Gvl. In this invention metal like Rh, Pt and Pd as catalysts were taken along with co-catalysts such as MnO2, BaO, and ZrO2 on carbon support. These catalysts showed good activity at 120 to 150° C., 10-100 bar H2 for 1 to 8 h. The drawbacks of the process are requirement of additional energy and use of raw materials as co-catalysts, and harsher reaction conditions.
A. M. R. Galletti et al., in their paper titled “A sustainable process for the production of γ-valerolactone by hydrogenation of biomass-derived levulinic acid” in Green Chem. (2012) 14, 688 disclosed Ru/C catalysts with the combination of various heterogeneous acid co-catalyst and found that Amberlyst A70 as co-catalyst showed higher yield (99.5%) at 70° C., 5 bar H2 within 3 h in aqueous medium. The drawbacks of the process are necessity of large amount of acid co-catalyst and long reaction time.
Y. Yang et al., in their paper titled “New route toward building active ruthenium nanoparticles on ordered mesoporous carbons with extremely high stability” in Scientific Reports (2014) 4, 4540 reported ruthenium nanoparticles on ordered mesoporous carbon as novel catalyst which showed high catalytic activity (99.4% LA conversion and 98.8% Gvl yield) at 150° C., 45 bar H2 within 2 h. Although their catalyst has high recycling ability, the drawbacks of the work are higher reaction temperature, pressure, and energy intensive synthesis of mesoporous carbon support that needed many additional chemicals.
C. V. Rode et al., in U.S. Pat. No. 8,975,421 B2 have disclosed recyclable Cu—ZrO2 nanocomposite as catalyst for maximum conversion of LA (100%) with 100% of Gvl selectivity at 200° C., 35 bar H2 for 5 h. Although this process claimed the use of inexpensive non-precious metal catalyst, harsh reaction conditions employed to achieve high yield is the main drawback of this process.
Globally, many research groups and industries reported several catalytic systems for selective conversion of LA to Gvl using hydrogen. However, till now no catalyst is available in the prior art that would enable this reaction with high yields under mild conditions (near room temperature and to the extent of hydrogen necessary) in shorter time (in less than 60 minutes) without using additional raw materials such as co-catalysts, ligands, chemicals involved in the preparation of catalysts.